A variety of sensors are utilized to detect conditions, such as pressure and temperature. The ability to detect pressure and/or temperature is an advantage to any device exposed to variable pressure conditions, which can be severely affected by these conditions. An example of such a device is a catheter, which of course, can experience variations in both temperature and pressure. Many different techniques have been proposed for sensing the pressure and/or temperature passively and wirelessly. These technologies include LC tank, RC or RLC oscillator type, acoustic wave sensor, and magneto-elastic sensor.
Acoustic wave devices often have one or two transducers arranged on a piezoelectric substrate. A transducer can convert an electrical input signal into an acoustic signal. The acoustic wave device can then alter the acoustic signal as it propagates through, or along, the substrate. A transducer can also convert the acoustic signal into an electrical signal. One common use of acoustic wave devices is conditioning or filtering signals, such as filtering the electrical signal received by a cellular telephone, because acoustic wave devices can be extremely good and inexpensive signal filters.
Acoustic wave devices are also used as sensors. The piezoelectric substrate is sensitive to environmental conditions. It can expand or contract with the temperature. Pressure can cause it to flex. Some chemical sensors are based on acoustic wave devices such that exposure to a chemical, such as water or alcohol, can introduce mass-loading effect. Mass-loading to the substrate of an acoustic wave device can change its acoustic properties. The changing acoustic properties can affect the acoustic signals which become electrical signals at the transducers. As such, acoustic wave devices have been used to measure temperature, pressure, chemical densities, and other environmental properties.
FIG. 4, labeled as “prior art”, illustrates one type of acoustic wave device known as a surface acoustic wave device (SAW). In a SAW the acoustic waves propagate along the surface of the substrate 401. The illustrated SAW has an input transducer 402, an output transducer 403, and a substrate 401. An input electrical signal enters the input transducer 402, becomes an acoustic signal, and travels over the substrate surface to the output transducer 403. The acoustic signal can be transformed by the substrate 401 as it travels over the substrate surface. The output transducer 403 then converts the acoustic signal into an output electrical signal. Essentially, the SAW transforms the input electrical signal into the output electrical signal.
FIG. 5, labeled as “prior art”, illustrates another type of acoustic wave device called a bulk acoustic wave device (BAW). In a BAW the acoustic signal travels through the substrate 501. A signal between the first lead 503 and the second lead 505 appears as a voltage difference between the first electrode 502 and second electrode 504. The voltage difference causes the substrate 501 to expand or contract. Similarly, expansions and contractions of the substrate 501, as can be caused by pressure, cause a voltage difference between the first electrode 502 and second electrode 504. The voltage difference then can then pass away from the BAW along the first lead 503 and the second lead 505.
Yet another type of sensor is the RLC sensor. RLC is shorthand for “resistor, inductor, and capacitor”. An RLC sensor is a sensor that, as with the acoustic wave devices discussed above, changes its fundamental frequency based on environmental conditions such as pressure or temperature. In some sensors, the resistive element changes with environmental conditions. In a resistive pressure sensor environmental pressure changes the resistance of at least one of the resistors. The changed resistance results in a changed fundamental frequency. Temperature can also cause resistance changes. A positive temperature coefficient (PTC) resistor increases resistance as the temperature increases. A negative temperature coefficient (NTC) resistor decreases resistance as the temperature increases.
Another use of PTC and NTC resistors is temperature compensation. For example, an RLC pressure sensor can have a capacitor that changes capacitance when pressure changes and when temperature changes. As such, the pressure sensor gives poor results is the temperature isn't constant and ideal. A temperature sensitive resistor can be used to compensate for the capacitor's temperature sensitivity. An RLC sensor that uses a pressure sensitive capacitor to sense pressure is a capacitive pressure sensor. An RLC sensor that uses a pressure sensitive inductor to sense pressure is an inductive pressure sensor.
An RLC sensor has no electrical components other than resistors, capacitors, and inductors. An LC sensor is a type of RLC sensor without resistors. An RC sensor is a type of RLC sensor without inductors. The term “LC tank” is synonymous with “LC” while the term “tank” can refer to either RLC or LC. Those skilled in the arts of analog electronics or analog sensors are familiar with using RLC, LC, RC, LC tank, and tank circuits as sensors and as passive sensors. They are also familiar with NTC and PTC resistors, the use of those resistors as sensing elements, and the use of those resistors as compensation elements.
Another sensor element is the magnetoelastic or magnetostrictive transducer. In a magnetoelastic or magnetostrictive transducer, the change in the position of a sensing shaft creates stress in the stress-sensitive core. The permeability of the core material alters with stress, effecting the inductance of the winding wound around the core. The inductance is a function of the shaft's position. The sensors fundamental frequency changes when the inductance changes. Those skilled in the art of sensors are familiar with the magnetoelastic or magnetostrictive transducer.
Many sensors can not be advantageously employed unless an interrogation circuit is also employed. These technologies include LC tank, RC or RLC oscillator type, acoustic wave sensor, and magneto-elastic sensor. An interrogation circuit is a circuit that creates an interrogation signal that is passed to the sensor. The sensor then returns a response signal or affects the interrogation signal in some way. For example, an electronic thermometer can accept an interrogation signal comprising power and ground while returning a voltage proportional to the temperature as a response signal. Alternatively, a material that changes its electrical resistance can be used as a temperature sensor. The interrogation signal can be power and ground voltages while the current passing through the circuit is the response signal.
FIG. 6, labeled as prior art, illustrates an inductively coupled interrogation circuit 601. A function generator 603 can produce a signal. One such signal is a repeating voltage ramp. Those skilled in the arts of electronics or signaling often call this a saw tooth signal. The saw tooth signal can be passed to a voltage controlled oscillator (VCO) 604. A VCO 604 produces a signal with a frequency dependent on an input voltage. Passing a saw tooth signal to the VCO 604 causes the VCO 604 to produce an interrogation signal that repeatedly sweeps through a range of frequencies. The interrogation signal is passed to an inductor 602, a grid dip oscillator (GDO) 108, and a phase locked loop (PLL) 605. The inductor 602 can inductively couple the interrogation signal into an inductive load (not shown).
The efficiency with which the interrogation signal is coupled into an inductive load depends on the inductive load and the interrogation signal frequency. Many circuits, including inductive loads, have a fundamental frequency. When the interrogation signal frequency matches the fundamental frequency, the coupling is maximized. As the interrogation signal sweeps through a frequency range, the fundamental frequency can be detected by the GDO 108 because the voltage across the inductor 602 drops to a minimum value at the inductive load's fundamental frequency. The GDO 108 signals the PLL 605 at the fundamental frequency. A PLL 605 is an oscillator that can lock onto and follow a source signal. As such, the PLL 605 locks onto and follows the interrogation signal. When signaled by the GDO 108, however, the PLL 605 can stop following the interrogation signal and remain producing a locked signal at the inductive load's fundamental frequency. The locked signal is passed to a frequency counter 606.
The system of FIG. 6 illustrates an interrogation circuit that can measure the fundamental frequency of an inductive load. Similar interrogation circuits are disclosed in U.S. Pat. Nos. 3,092,806 and 3,906,340. U.S. Pat. Nos. 3,092,806 and 3,906,340 are incorporated by reference into this document.
Some acoustic wave devices can inductively couple with an interrogation circuit. However, high inductive coupling efficiency and high sensor sensitivity can be competing design goals. Devices designed for coupling efficiency can lack sensitivity. Sensitive devices can couple poorly. A driving element, or driver, can improve the sensor. The driver is electrically connected to the acoustic wave device and inductively coupled to interrogation circuit. The problem, however, is that the acoustic wave device and the driver can both inductively couple with the interrogation circuit. This can lead to a noisy sensor reading. Another concern is that the driver and the acoustic wave device can inductively couple with one another, creating another source of noise in the sensor readings.
Spiral inductors are inductors that are formed on a planar substrate. They have been formed and used on single layer circuit boards and, in some specialized circumstances, in integrated circuits. Those skilled in the arts of radio frequency circuits, circuit layout, or inductors are aware of the design and use of spiral inductors.
Aspects of the embodiments directly address the shortcoming of current technology by using spiral inductors as drivers and using physical and geometric mechanisms to shield the acoustic wave device from various sources of inductive coupling.